Method for making brazed heat exchanger and apparatus

ABSTRACT

A heat exchanger has a boiling passage and a cooling passage defined by opposite sides of metal walls. Layers of brazing material between the metal walls and a spacer member bond components of the heat exchanger together. An enhanced boiling layer (EBL) having metal particles bonded to each other and to a boiling side of the metal wall provides nucleate boiling pores to improve heat transfer. The EBL has a melting temperature that is higher than the melting temperature of the brazing material.

FIELD OF THE INVENTION

[0001] This invention relates to an improved method for making a metalheat exchanger with high heat transfer efficiency. Specifically, thisinvention relates to an improved method for making a brazed heatexchanger containing enhanced boiling surfaces.

BACKGROUND OF THE INVENTION

[0002] Two designs of heat exchanger are presently in general use forreboiler-condensers in cryogenic, refinery and chemical applications.One type of heat exchanger in current use is a vertical shell and tubeheat exchanger. To achieve a sufficiently high degree of heat transferat relatively low temperature differences with this design, enhancedboiling layers (EBL) are used. An EBL typically has a structurecomprising a multitude of pores that provide boiling nucleation sites tofacilitate boiling. An EBL is applied to the inside of the tubes, andlongitudinal flutes are provided on the outside of the tubes tofacilitate heat transfer.

[0003] Enhanced boiling layers were first proposed for heat exchangersin U.S. Pat. No. 3,384,154. This patent discloses mixing metal powder ina plastic binder in solvent and applying the slurry to a base metalsurface. The coated metal is subjected to a reducing atmosphere andheated to a temperature for sufficient time so that the metal particlessinter together and to the base metal surface. U.S. Pat. No. 3,457,990discloses an enhanced boiling surface with reentrant groovesmechanically or chemically formed therein.

[0004] Other methods of applying EBLs have been disclosed. GB 2 034 355discloses applying an organic foam layer to a metal heat transfer memberand plating the foam with metal such as copper first by electroless,then by electrodeposition. U.S. Pat. No. 4,258,783 disclosesmechanically forming indentations in a heat transfer surface and thenelectrodepositing metal on the pitted surface. GB 2 062 207 disclosesapplying metal particles to a metal base by powder flame spraying. EP303 493 discloses spraying a mixture of metal and plastic material ontoa base metal by flame or plasma spraying. U.S. Pat. No. 4,767,497 andU.S. Pat. No. 4,846,267 disclose heat treating an aluminum alloy plateto produce a precipitate followed by chemically etching away theprecipitate to leave a pitted surface. EP 112 782 discloses applying amixture of brazing alloy and spherical particles to a metallic wall andheating the coated wall to melt the brazing material.

[0005] A common heat exchanger used in cryogenic, refinery and chemicalapplications is the plate-fin brazed aluminum heat exchanger fabricatedby disposing corrugated aluminum sheets between aluminum parting sheetsor walls to form a plurality of fluid passages. The sheets are eitherclad with an aluminum brazing layer or a layer of brazing foil isinserted between the surfaces to be bonded. When heated to apredetermined temperature for a predetermined period of time, thebrazing foil or cladding melts and forms a metallurgical bond with theadjacent sheets. The resulting heat exchanger contains numerous passagesconsisting of alternate layers of closely spaced fins. A typicalarrangement of alternate layers of passages each containing fins with adensity of 6 to 10 fins/cm (15 to 25 fins/inch), and a fin height of 0.5to 1 cm (0.2 to 0.4 inch). In a common application, a first series ofalternating passages carry vapor for condensing, while a second seriesof alternating passages carry a liquid for boiling. Typical brazedaluminum heat exchangers must be able to withstand 2068 to 2758 kPa (300to 400 psia).

[0006] Patents proposing replacing fins with an enhanced boiling layerin the boiling passages of a brazed heat exchanger include U.S. Pat. No.5,868,199; U.S. Pat. No. 4,715,431 and U.S. Pat. No. 4,715,433. Thesepatents propose to stack aluminum sheets each with an EBL applied on oneside to define boiling channels and with fins on the other side of thealuminum sheets to define condensing channels. Layers of brazingmaterial are disposed between bonding surfaces in the stack, and thestack is subjected to heating over a period of time to obtain a brazedheat exchange core. Such brazed aluminum heat exchangers described inthese patents have not been commercialized EBLs are typically brazed at565° to 593° C. (1050° to 1100° F.) while the subsequent brazing of themetal components together occur at around 593° to 621° C. (1100° to1150° F.). Maintaining the integrity and effectiveness of the EBL,particularly the porous structure provided by the mutually bonded metalparticles, during the second hotter heat treatment to effect brazing hasbeen difficult. This difficulty accounts for the lack of commerciallyavailable brazed heat exchangers with EBL in the boiling passages.

SUMMARY OF THE INVENTION

[0007] The present invention is an improved method for making a brazedmetal heat exchanger and the resulting apparatus. An enhanced boilinglayer (EBL) is provided on the walls of the boiling passages. Themelting temperature of the brazing material is lower than the meltingtemperature of the metal particles in the enhanced boiling layer. In anembodiment, the metal in the enhanced boiling layer and/or the brazinglayer is an alloy of a first metal and a second metal which alloy has alower melting temperature than that of the first metal. Different secondmetals can be used in the EBL and in the brazing material so long as thesecond metal provides an alloy with a lower melting temperature. In anembodiment, the concentration of the second metal in the brazingmaterial is greater than in the EBL. Hence, we have found that even whenthe brazing temperature gets within about 8.3 Celsius degrees (15Fahrenheit degrees) of the melting point of the metal in the EBL for anextended period of time, the EBL unexpectedly retains its porosity, andthus its effectiveness. In an embodiment, the condensing passagescontain fins to facilitate heat transfer.

[0008] An object of the present invention is to provide a metal heatexchanger with EBLs in the boiling passages with undiminished heattransfer capability despite being subjected to brazing temperatureduring manufacturing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a perspective view of three heat exchangers.

[0010]FIG. 2 is a perspective view of the core of a heat exchanger inFIG. 1 with layers broken away to reveal internals.

[0011]FIG. 3 is a perspective view of the core of the heat exchanger inFIG. 1 but taken from a different perspective than FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The methods of the present invention can be used to construct anyconfiguration of heat exchanger by brazing including shell and tube butmay be most appropriately applied to plate exchangers. The boiling andcooling passages of the heat exchangers of the present invention may beoriented to provide cross flow, counter-current flow or cocurrent flow.Moreover, the heat exchanger of the present invention may be applied inthe context of cryogenic air separation, hydrocarbon processing or anyother process that relies on boiling to effect heat exchange. Severaltypes of metals can be used for construction of heat exchangers.Aluminum is the most widely used metal for brazed heat exchangers.Aluminum is suitable for cryogenic applications because it resistsembrittlement at lower temperatures. Steel or copper may be used forheating or cooling fluids that may be corrosive to aluminum. Forpurposes of illustration, the present invention will be described withrespect to a counter current, aluminum, plate heat exchanger useful inthe context of cryogenic air separation.

[0013]FIG. 1 shows a train of typical plate heat exchangers 10 used incryogenic air separation. The heat exchangers 10 have alternatingboiling passages 12 and cooling passages 14 provided in a core 20. Aliquid such as liquid oxygen is delivered by conduits 16 to manifolds 18and distributed to the boiling passages 12. Delivery of liquid to theboiling passages 12 by means other than the conduits 16 or the manifolds18 underneath the core 20 is contemplated such as by thermosiphoning atthe bottom of the boiling passages 12. Moreover, liquid may be deliveredto the boiling passages 12 from the side or from the top of the core 20,perhaps through a distribution network that may comprise distributorfins. The liquid boils in the boiling passages 12, thereby indirectlywithdrawing heat conducted from the cooling passages 14. Gaseous oxygenfrom the boiling passages 12 are collected such as by headers 22 andremoved through a conduit 24. Collection of gases from the boilingpassages 12 by means other than the conduits 24 or the headers 22 abovethe core 20 is contemplated such as may be provided in a thermosiphoningarrangement. Moreover, gases may be collected from the boiling passages12 from the side or from the top of the core 20, perhaps through acollection network that may comprise collection fins. A fluid such asgaseous nitrogen is delivered by conduits 26 to manifolds 28 anddistributed to the cooling passages 14. Delivery by means other than bythe conduits 26 or the manifolds 28 is also contemplated. A liquid orgas can be cooled in the cooling passages 14. Moreover, if a gas isdelivered to the cooling passages 14, it may be cooled to such extent toeffect a phase change with or without temperature change depending onthe needs of the process. Heat conducted across the walls between thecooling passages 14 and the boiling passages 12 to support the boilingin the boiling passages 12 cools the fluid in the cooling passages 14,thereby condensing the nitrogen gas in the case of air separation. Fluidsuch as liquefied nitrogen from the cooling passages 14 is collectedsuch as by headers 30 and removed through conduits 32. Collection ofcooled fluid from the cooling passages 14 by means other than theheaders 30 and the conduits 32 is contemplated. Moreover, the deliveryand collection manifolds and conduits shown in the embodiment in FIG. 1may be modified and remain within the scope of the present invention.

[0014]FIG. 2 shows the core 20 of one of the heat exchangers 10 withparts broken away to reveal internals. A cap sheet 40 is disposed onboth ends of the core 20 to define the last channel on each end. Part ofthe cap sheet 40 illustrated in FIG. 2 is broken away to reveal theboiling passage 12. Vertical spacer bars or spacer members 42 aredisposed between opposing edges of the cap sheet 40 and a metal wall 44with a boiling side 44 a covered with an enhanced boiling layer (EBL)46. The EBL 46 comprises thermoconductive particles bonded to theboiling side 44 a and to each other to form a texture of pores in whichnucleate boiling sites are provided. The thermoconductive particles aremetal particles in an embodiment. Hence, the boiling passage 12 isdefined by an inner surface of the cap sheet 40, inner edges of thevertical spacer bars 42 and the boiling side of the metal wall 44. Outervertical margins 48 of the boiling side 44 a are devoid of the EBL 46 toprovide a bonding surface. Vapor leaves the boiling passages 12 throughboiling outlets 49, which may be collected by the boiling headers 22,shown in the embodiment of FIG. 1. Moreover, it is contemplated that theboiling passages 12 may contain fins to further facilitate heattransfer. Behind the broken away metal wall 44 and the vertical spacerbars 42 is the cooling passage 14 including primary fins 52 comprising acorrugated sheet of a primary fin stock 54. The primary fins 52 extendlaterally between inner edges of the vertical spacer bars 42 at oppositeends of the cooling passage 14. Distributor fins 56 comprising adistributor fin stock 58 or being integral with the primary fin stock 54are disposed in an inclined configuration to evenly distribute coolingfluid from cooling inlets 50 along the tops of the channels provided bythe primary fins 52. In the embodiment of FIG. 2, cooling fluid isreceived into cooling inlets 50 which may come from the cooling manifold28 as shown in the embodiment of FIG. 1. Another type of distributionconfiguration with or without fins may be used to distribute coolingfluid. In another embodiment, the cooling inlets 50 may be consideredthe tops of the channels provided by the primary fins 52. For purposesof illustrating the tops of the primary fins 52, only one set of thedistributor fins 56 is shown in FIG. 2. Cooling outlets 64 which may bedefined by collection fins 66 allow cooled fluid to exit the core 20. Inthe embodiment of FIG. 2, cooling fluid exits through cooling outlets 64which may enter into the cooling header 30 in the embodiment of FIG. 1.Horizontal spacer bars 60 seal the top and the bottom of the coolingpassages 14. The spacer bars 42, 60 and the fins 52, 56, 66 space acooling side 44 b (the opposite side) of the metal wall 44 from thecooling side 44 b of the adjacent metal wall 44. In an embodiment, nohorizontal spacer bars 60 are provided in the boiling passages 12 topermit entry and exit of fluid to and from the boiling passages 12,respectively. Hence, the vertical spacer bars 42 are sandwiched betweenopposite ends of each pair of the adjacent metal walls 44, while thehorizontal spacer bars 60 are sandwiched only between the adjacentcooling sides 44 b. However, if the fins 52, 56, 66 are arranged andbonded appropriately to withstand operating pressure, it is contemplatedthat spacer bars 42, 60 can be omitted between the cooling sides 44 b inthe cooling passage 14. Hence, the fins 52, 56, 66 would provide thespacing function. The walls 44 have an alternating orientation. Exceptwhen adjacent to the cap sheet 40, the cooling side 44 b of the metalwall 44 is always facing the cooling side 44 b of an adjacent wall, andthe boiling side 44 a of a wall is always facing the boiling side 44 aof the adjacent metal wall 44. It is also contemplated in embodimentsthat the cooling passages 14 include no fins and that the boilingpassages 12 be equipped with fins.

[0015]FIG. 3 shows the core 20 of FIG. 2 but from a perspective thatshows the bottom of the core 20. All elements in FIG. 2 that are visiblein FIG. 3 are referenced with numerals. Additionally, boiling inlets 51to the boiling passages 12 are shown. In an embodiment, the boilinginlets 51 may receive boiling liquid from boiling manifolds 18 (FIG. 1).Moreover, the bottom of the cap sheet 40 and the first metal wall 44 arebroken away to reveal collection fins 66 from a third fin stock 68. Thecollection fins 66 comprising the third fin stock 68 or being integralwith the primary fin stock 54 are disposed in an inclined configurationto evenly collect cooling fluid from cooling outlets 64 along thebottoms of the channels provided by the primary fins 52. Another type ofcollection configuration with or without fins may be used to collectcooling fluid. In another embodiment, the cooling outlets 64 may beconsidered the bottoms of the channels provided by the primary fins 52.For purposes of illustrating the bottoms of the primary fins 52, onlyone set of the collection fins 66 is shown in FIG. 3.

[0016] The EBL is added to the boiling side by any of the methods knownin the art, such as by applying a slurry, flame spraying, plasmaspraying or by electrodeposition. However, it is critical that thesubsequent brazing step not diminish the heat exchange efficiency of theEBL once applied. In an embodiment, the melting point of the EBL ishigher than the melting point of the brazing metal. The relative meltingpoints of the brazing metal and EBL may be obtained by alloying a secondmetal with a first metal that has the effect of providing a meltingpoint of the alloy that is lower than the melting point of the firstmetal. The concentration of the second metal may be higher in thebrazing metal than in the EBL material, so that the EBL has a highermelting point that can withstand the brazing step without loss ofstructural integrity. In brazed aluminum heat exchangers, aluminum isthe first metal and silicon, manganese, magnesium or alloys thereof maybe the second metal. In brazed steel heat exchangers, nickel may be thefirst metal and phosphorous may be the second metal. In brazed copperheat exchangers, copper may be the first metal and phosphorous may bethe second metal.

[0017] In the case of copper being the first metal used to provide theEBL and the brazing material, brazing occurs at about 100° C. (180° F.)below the melting temperature of copper or at about 960° C. (1760° F.).In the case of aluminum being the first metal, brazing occurs at about49° to 54° C. (120° to 130° F.) below its melting temperature of about649° C. (1200° F.). If nickel is the first metal, the brazing step inthe furnace will take place at a temperature of about 1037° C. (1900°F.) which is 38° C. (100° F.) below the melting temperature of steel. Atthese temperatures, the second metal lowers the melting point of thealloy with the first metal. The liquefied brazing metal flows anddiffuses into the base metal and forms a metallurgical bond. By alloyingmore of the second metal with the first metal in the braze material thanin the EBL material, the EBL once applied will be able to withstand thesubsequent lower temperature brazing heat treatment.

[0018] It is also contemplated that sintering may be used to form theEBL instead of brazing. In sintering, the metal is heated to the pointof molecular agitation and diffuses over a relatively long period oftime into an adjacent metal to form metallurgical bonds. Sintering maybe used to provide the EBL with brazing at a lower temperature to bondthe components of the heat exchanger together.

[0019] In an embodiment, the first step of applying the EBL is applyinga polymer binder to the boiling side of the metal wall. A metal powderwhich may comprise the first metal and the second metal are thensprinkled onto the plastic binder. The metal wall with metal powderbound by the plastic thereto is blanketed with an inert atmosphere suchas nitrogen and the temperature is raised to a brazing temperature forsufficient time to effect metallurgical bonds between the metal powderparticles to each other and to the boiling side of the metal wall. Theplastic binder decomposes under heat and evaporates. The circulatinginert gas diminishes formation of an oxide film and also purges thedecomposition gases from the binder material. The bonded metal powderforms a highly porous, three-dimensional matrix that provides the EBLwith nucleate boiling sites.

[0020] Appropriate plastic binders include polyisobutylene,polymethylcellulose having a viscosity of at least 4000 cps and soldcommercially as METHOCEL and polystyrene having a molecular weight of90,000. The binder may be dissolved in an appropriate solvent such askerosene or carbon tetrachloride for polyisobutylene andpolymethylcellulose binders and xylene or toluene for polystyrenebinder. The boiling side should be cleaned to be free of grease, oil oroxide to obtain proper bonding of the EBL thereto. Before applying theplastic solution, the boiling side may be flushed with the plasticsolution to facilitate wetting, thereby obtaining a more evendistribution of plastic binder. The plastic solution may be applied tothe boiling side in a way that will achieve a uniform layer such as byspraying, dipping, brushing or paint rolling. After application, thelayer is air dried either during or after the application of the metalpowder to evaporate away most of the solvent. A solid, self-supportinglayer of metal powder and binder is left in place on the metal wall bythe binder.

[0021] The metal powder comprising the first and second metal are mixedwith a flux. Upon heating, the flux melts and draws oxides from themetal which could inhibit the bonding of the metal particles to eachother and to the boiling side. The flux may be a mineral salt such ascommercially available potassium aluminum fluoride, which is a mixtureof KAlF₄ and KAlF₆. Other fluxes may be suitable.

[0022] The core 20 of the heat exchanger 10 is assembled by stackinglayers of components. If the brazing of the core 20 will not beperformed in a vacuum furnace, each component should be coated with fluxbefore stacking. A suitable way to coat components with flux componentsis to mix the flux with denatured alcohol in 1:1 volumetric ratio andbrush or spray the flux solution onto the component before stacking. Theorder of stacking will be described with the side shown in FIGS. 2 and 3on the bottom. The cap sheet 40 is placed on the bottom of a stackingsurface with the outer surface of the cap sheet 40 down. A layer ofbrazing foil is layered at least on the two vertical margins 48 of aninner surface of the cap sheet 40 or perhaps over the whole innersurface of the cap sheet 40. The vertical spacer bars 42 are stacked onthe vertical margins 48 of the inner surface of the cap sheet 40. Thebrazing foil may be provided only at the vertical margins 48 of the capsheet 40 because only the vertical spacer bars 42 will be brazed to theinner surface of the cap sheet 40 that is defining the boiling passage12 in this case. Typically, no horizontal spacer bars 60 are stacked inthe boiling passage 12. However, in an embodiment, if the cap sheet 40is defining the cooling passage 14, the horizontal spacer bars 60 shouldbe stacked on and brazed to the cap sheet 40. A layer of brazing foil isstacked on top of the vertical spacer bars 42. Strips of the brazingfoil may be placed just over the vertical spacer bars 42. The metal wall44 with the EBL 46 on the boiling side 44 a facing downwardly toward thecap sheet 40 and the cooling side 44 b facing upwardly is stacked on topof the vertical spacer bars 42. The vertical margins 48 of the boilingside 44 a which are devoid of the EBL 46 will rest on the brazing foilon top of the vertical spacer bars 42. A layer of brazing foil is laidon top of the cooling side 44 b of the metal wall 44. The primary finstock 54 comprising the primary fins 52, the distributor fin stock 58comprising the distributor fins 56, the collection fin stock 68comprising the collection fins 66 and the horizontal spacer bars 60 andthe vertical spacer bars 42 are all stacked on top of the layer ofbrazing foil laid on top of the cooling side 44 b of the metal wall 44.A layer of brazing foil is laid upon the primary fin stock 54, thedistributor fin stock 58, the collection fin stock 68 comprising thecollection fins 66 and the spacer bars 42, 60. Next, another metal wall44 with the cooling side 44 b facing downwardly and the boiling side 44a facing upwardly is laid upon the layer of brazing foil. On the top ofthe metal wall 44, strips of brazing foil are laid down just in thevertical margins 48 of the boiling side 44 a outside of the EBL 46. Thevertical spacer bars 42 are laid down on top of the strips of brazingfoil in the vertical margins 48. Strips of brazing foil are laid on topof the vertical spacer bars 42. An additional metal wall 44 with theboiling side 44 a facing downwardly is stacked on top with the verticalmargins 48 mating with the strips of brazing material on top of thevertical spacer bars 42. The rest of the core 20 of the heat exchanger10 is stacked as previously described until the cap sheet 40 is stackedon the top of the stack. It is also contemplated that both sides of theprimary fin stock 54, the spacer bars 42, 60 and/or the cooling side 44b of the metal wall 44 may be integrally clad with a layer of brazingmaterial. This would obviate the need for adding layers of brazing foilin the stack constituting the core 20. However, if just the fin stock54, 58, 68 and/or the spacer bars 42, 60 can be obtained with brazedmaterial clad on both sides, the use of brazing foil may be obviated.

[0023] After the core 20 is fully stacked it is inserted into a furnacewith an atmosphere of inert gas and heated so that the center 20 of thecore attains an elevated temperature. After remaining at the elevatedtemperature for a period of time, it is allowed to cool. The elevatedtemperature is above the melting temperature of the brazing material andbelow the melting temperature of the EBL 46 material upon applicationand the melting temperature of the base metal. In an embodiment, theelevated temperature may be below the melting temperature of the EBL 46material after application. In a controlled atmosphere brazingenvironment, Aluminum Alloy 4047 may be used for the brazing material inwhich case the elevated brazing temperature would be approximately 607°to about 618° C. (1125° to 1145° F.). Aluminum alloy designations givenherein will be pursuant to the convention of alloys used by those ofordinary skill in the art of aluminum brazing. The brazing materialmelts and forms a metallurgical bond with adjacent metal members toprovide a robust metal heat exchanger core. The EBL 46 maintains itshighly porous structural integrity. Residues of flux on the surface ofthe core 20 may remain but will typically wash out without affectingoperation.

[0024] After brazing the core 20 together, the manifolds 18, 28 and theheaders 22, 30 are welded to the core 20 as shown in the embodiment inFIG. 1. The conduits 16, 24, 26, 32 are all affixed to the appropriatemanifold 18, 28 or the header 22, 30. Other delivery, distribution,collection and recovery equipment than shown in the embodiment of FIG. 1may be used within the scope of the present invention.

[0025] Alternatively, one or both of the brazing steps may take place ina vacuum oven. Flux becomes unnecessary and a lower temperature istypically used for brazing. However, in the vacuum brazing process, ittakes longer for the core to reach the brazing temperature, after which,cooling is allowed. If the stacked core is brazed in a vacuumenvironment, Aluminum Alloy 4104 may be used for brazing material inwhich case the elevated brazing temperature would be approximately 582°to about 593° C. (1080° to 1100° F.).

[0026] It is important, for purposes of this invention, that the EBL beable to withstand the final brazing heat treatment. In a brazed aluminumheat exchanger, brazing material, whether it be powder, foil or claddingmay comprise a eutectic alloy of at least about 80 wt-% aluminum andabout 10 to about 15 wt-% silicon. In an embodiment, the eutectic alloycomprises about 11 to about 13 wt-% silicon and at least about 85 wt-%aluminum. In a further embodiment, the brazing eutectic alloy may beAluminum Alloy 4047 and comprise about 12 wt-% silicon and about 88 wt-%aluminum. Other components of the core 20, such as the walls, the finstock and the spacer bars may comprise Aluminum Alloy 3003 whichcomprises a highly proportioned aluminum alloy of as low as about 98wt-% aluminum and as high as about 2 wt-% manganese. Small amounts ofmagnesium and iron may also be present in Aluminum Alloy 3003. The term“highly proportioned” means greater than 90 wt-%. Other componentscomprising substantially pure aluminum or highly proportioned aluminumalloys may be suitable. In vacuum brazing applications, about 1 to 2wt-% of magnesium may be provided in the highly proportioned aluminumalloy. The material comprising the EBL may comprise about 0.5 to about1.5 wt-% silicon and at least about 95 wt-% substantially pure aluminumor highly proportioned aluminum alloy. In an embodiment, the EBL maycomprise about 5 to about 11 wt-% brazing material and at least about 85wt-% substantially pure aluminum or highly proportioned aluminum alloy.In an embodiment, the EBL comprises at least about 90 wt-% pure orhighly proportioned aluminum and a eutectic alloy including about 11 toabout 13 wt-% silicon and at least about 85 wt-% aluminum. In anembodiment, the eutectic alloy in powder form is mixed with powderedsubstantially pure or highly proportioned aluminum. To prevent oxidationof the aluminum in nonvacuum brazing ovens, a flux comprising about 5 toabout 10 wt-% of a powdered mineral salt should be included in the EBLmaterial upon application.

[0027] While not wishing to be bound to any particular theory, webelieve that upon heating, a powdered EBL material mixture describedabove, the brazing eutectic alloy powder melts and wets the solid,unmelted substantially aluminum powder, thereby forming an alloy. Webelieve that after application, the resulting alloy in the EBL melts ata higher temperature than the brazing eutectic alloy by virtue of thelower concentration of the silicon metal in the aluminum alloy. The EBLis then able to withstand brazing temperatures associated with bondingthe stacked heat exchanger core that are perilously close to thetemperature at which the EBL material was initially brazed without lossof performance.

[0028] If the EBL is sintered, pure Aluminum Alloy 3003 powder may besintered at about 1185° F. (641° C.). Brazing foil comprising theeutectic of silicon and aluminum mentioned above may be used to bond thecore together at a brazing temperature of about 60° to 613° C. (1120° to1135° F.) under a controlled inert atmosphere and a brazing temperatureof about 566° to 596° C. (1050° to 1105° F.) in a vacuum environment.

EXAMPLE I

[0029] An enhanced boiling powder was obtained by mixing 83.6 wt-%Aluminum Alloy 3003 powder, 8.4 wt-% brazing flux comprising potassiumaluminum fluoride and 8.0 wt-% Aluminum Alloy 4047 brazing powder. Anadhesive comprising 38 wt-% polyisobutylene sold as CS-200 A3 by CliftonAdhesives and 62 wt-% VARSOL light kerosene solvent was mixed andbrushed onto three tubular walls comprising Aluminum Alloy 3003. Theenhanced boiling powder was then sprinkled onto the adhesive and heatedunder nitrogen in a small furnace. Each coated tubular wall was heatedto 621° C. (1150° F.) for nine minutes. The adhesive and solventevaporated off, leaving an EBL of about 0.3 to 0.4 millimeters (10 to 15mils) thick. The resulting EBL had a highly porous structure and wasdetermined to have boiling heat transfer coefficients above 204,418kJ/hr/m²K (10,000 BTU/hr/ft²° F.).

EXAMPLE II

[0030] Two metal tubular walls were coated with the adhesive and theenhanced boiling powder as explained in Example I. Each tubular wall washeated in a controlled nitrogen atmosphere to a brazing temperature of623° C. (1153° F.) in a closed retort at about atmospheric pressure andthen allowed to cool.

[0031] A first tubular metal wall was heated and cooled over a period of48 minutes. The first tubular metal wall was tested and determined tohave a heat transfer coefficient of above 204,418 kJ/hr/m²K (10,000BTU/hr/ft²/° F.), which is more than adequate for a surface with an EBL.The first tubular metal wall was then subjected to a second furnacing tosimulate vacuum brazing of an entire heat exchanger core by heating itto a temperature of 593° C. (1100° F.) and allowing it to reside at thattemperature over a twenty-four hour period before cooling. Visualinspection revealed that the quality of the EBL was not impacted. Thefirst tubular metal wall was again tested and determined to have a heattransfer coefficient of above 204,418 kJ/hr/m²K (10,000 BTU/hr/ft²/°F.).

[0032] A second tubular metal wall was heated and cooled over a periodof 36 minutes. The second tubular metal wall was tested and determinedto have a heat transfer coefficient of above 204,418 kJ/hr/m²K (10,000BTU/hr/ft²/° F.), which is adequate for a surface with an EBL. Thesecond tubular metal wall was then subjected to a second furnacing tosimulate controlled atmosphere brazing of an entire heat exchanger coreby heating it to a temperature of 613° C. (1135° F.) and allowing it toreside at that temperature over a two hour period under nitrogen atatmospheric pressure before cooling. Visual inspection revealed that thequality of the EBL was not impacted. The second tubular metal wall wasagain tested and determined to have a boiling heat transfer coefficientof above 204,418 kJ/hr/m²K (10,000 BTU/hr/ft²° F.). After heating theEBL to a temperature of 8.3 Celsius degrees (15 Fahrenheit degrees) fromthe brazing temperature of the EBL, the structure of the EBL withstoodthe heat treatment without noticeable loss to structure or performance.

1: A heat exchanger comprising: a plurality of metal walls, each metal wall comprising two sides, a boiling side with an enhanced boiling layer comprising thermally conductive particles integrally bonded together and metallurgically bonded to the boiling side and a cooling side, said boiling side of said plurality of metal walls defining a boiling passage and said cooling side of said plurality of metal walls defining a cooling passage and each of said plurality of metal walls further including a bonding surface; a spacer member for spacing metal walls from each other; a layer of metal between said bonding surfaces of said metal walls and said spacer member in said heat exchanger, said layer of metal having a melting temperature that is less than a melting temperature of said enhanced boiling layer; a boiling inlet for delivering liquid to said boiling passage; a cooling inlet for delivering fluid to said cooling passage; a boiling outlet for recovering vapor from said boiling passage; and a cooling outlet for recovering fluid from said cooling passage. 2: The heat exchanger of claim 1 wherein the metal walls predominantly comprise aluminum. 3: The heat exchanger of claim 1 wherein the thermally conductive particles predominantly comprise aluminum. 4: The heat exchanger of claim 3 wherein said enhanced boiling layer includes between about 0.5 and about 1.5 wt-% silicon. 5: The heat exchanger of claim 3 wherein said thermally conductive particles of said enhanced boiling layer comprises a highly proportioned aluminum alloy powder mixed with a eutectic alloy of aluminum and silicon. 6: The heat exchanger of claim 5 wherein the highly proportioned aluminum alloy comprises 92 wt-% of the enhanced boiling layer and the eutectic alloy comprises 8 wt-% of the enhanced boiling layer. 7: The heat exchanger of claim 5 wherein said layer of metal comprises said eutectic alloy. 8: The heat exchanger of claim 5 wherein the eutectic alloy is 12 wt-% silicon and 88 wt-% aluminum. 9: A heat exchanger comprising: a plurality of metal walls, each metal wall comprising two sides, a boiling side with an enhanced boiling layer comprising thermally conductive particles integrally bonded together and a cooling side, said thermally conductive particles comprising an alloy of a first metal and a second metal, said second metal alloying with said first metal to provide an alloy with a melting temperature that is lower than the melting temperature of said first metal, said boiling sides of said plurality of metal walls defining boiling passages and said cooling sides of said plurality of metal walls defining cooling passages and each of said plurality of metal walls further including a bonding surface; a plurality of spacer bars each between pairs of said metal walls, each of said spacer bars having a bonding surface; a layer of metal between each of said bonding surfaces of said metal walls and a bonding surface of an adjacent one of said plurality of spacer bars, said layer of metal comprising an alloy including said first metal and a melting temperature of said layer of metal is less than a melting temperature of said enhanced boiling layer; a boiling inlet for delivering liquid to said boiling passage; a cooling inlet for delivering fluid to said cooling passages; a boiling outlet for recovering vapor from said boiling passages; and a cooling outlet for recovering fluid from said cooling passages. 10: The heat exchanger of claim 9 wherein said layer of metal comprises an additional metal with a greater concentration of said additional metal than a concentration of said second metal in said enhanced boiling layer. 11: The heat exchanger of claim 9 wherein the metal walls and the first metal predominantly comprise aluminum. 12: The heat exchanger of claim 9 wherein the second metal and the additional metal are silicon. 13: The heat exchanger of claim 9 wherein the enhanced boiling layer includes between 0.5 and 1.5 wt-% silicon. 14: The heat exchanger of claim 9 wherein the thermally conductive particles of said enhanced boiling layer comprises a highly proportioned aluminum alloy powder mixed with a eutectic alloy of aluminum and silicon 15: The heat exchanger of claim 14 wherein the highly proportioned aluminum alloy comprises 92 wt-% of the enhanced boiling layer and the eutectic alloy comprises 8 wt-% of the enhanced boiling layer. 16: The heat exchanger of claim 15 wherein said eutectic alloy is 12 wt-% silicon and 88 wt-% aluminum. 17-18 (canceled) 19: A method of constructing a heat exchanger comprising: providing a plurality of metal walls having a boiling side, a cooling side and at least one bonding surface; applying thermally conductive particles to a boiling side of a plurality of metal components; heating said metal walls with applied thermally conductive particles to a first temperature to integrally bond said thermally conductive particles together and to metallurgically bond thermally conductive particles to said boiling side to form an enhanced boiling surface, assembling said plurality of metal walls with a spacing member so that said boiling sides of said plurality of metal walls define a boiling passage and said cooling sides of said plurality of metal walls define a cooling passage and providing layers of metal between said bonding surfaces of said metal walls and adjacent surfaces of the spacing member; and heating the assembly to a second temperature that is less than said first temperature to bond the layers of metal to at least one of the adjacent surfaces of the spacing member and the bonding surfaces of the metal walls. 20: The method of claim 19 further comprising: affixing a boiling header to the heat exchanger to be in fluid communication with an inlet to said boiling passages; affixing a cooling header to said heat exchanger to be in fluid communication with an inlet to said cooling passages; affixing a boiling manifold to said heat exchanger to be in fluid communication with an outlet of said boiling passages; and affixing a cooling manifold to said heat exchanger to be in fluid communication with an outlet of said cooling passages. 21: A heat exchanger comprising: a plurality of metal walls, each metal wall comprising two sides, a cooling side and a boiling side with an enhanced boiling layer comprising thermally conductive particles, said thermally conductive particles including a highly proportioned aluminum alloy powder mixed with a eutectic alloy of aluminum and silicon, said thermally conductive particles being integrally bonded together and metallurgically bonded to the boiling side, said boiling side of said plurality of metal walls defining a boiling passage and said cooling side of said plurality of metal walls defining a cooling passage and each of said plurality of metal walls further including a bonding surface; a spacer member for spacing metal walls from each other; a layer of metal between said bonding surfaces of said metal walls and said spacer member in said heat exchanger, said layer of metal having a melting temperature that is less than a melting temperature of said enhanced boiling layer; a boiling inlet for delivering liquid to said boiling passage; a cooling inlet for delivering fluid to said cooling passage; a boiling outlet for recovering vapor from said boiling passage; and a cooling outlet for recovering fluid from said cooling passage. 